EP0221514A2 - Method of producing a grid pattern incorporating a phase jump on the surface of a substrate - Google Patents

Abstract

A method for producing a lattice structure with a phase jump on a surface of a substrate by exposure of a photosensitive surface in an optical interference field is described, the photosensitive surface being exposed twice and the spatial frequency of the interference field being changed by a small amount between the two exposures. The photosensitive surface can be a photoresist layer applied to the substrate. After the exposed layer has been developed, the lattice structure is produced by etching. The structure can also be produced directly by laser-activated etching, the photosensitive surface consisting of the substrate surface in contact with an etching agent, on which the etching reaction can be activated by exposure.

Description

The invention relates to a method for producing a lattice structure with a phase jump on a surface of a substrate according to the preamble of patent claim 1.

In order to achieve high data rates on long transmission paths, special semiconductor lasers are required for optical communications technology, which emit in a single longitudinal oscillation mode even at high modulation frequencies. A fundamentally suitable type of laser with distributed feedback (DFB lasers, DFB is the abbreviation for "d istributed f eed b ack), in which the feedback of the light in the laser resonator not by two mirrors but by the entire laser structure superimposed In general, however, a DFB laser oscillates not only in one mode, but in two modes, but single-mode light emission can be enforced, among other things, by dividing the lattice structure into two sub-gratings, the phase of which is half a grating constant, ie one Such a phase shift DFB laser is described, for example, in HA: Haus, CV Shank: Antisymmetric Taper of Distributed Feedback Laser, IEEE J. Quant. Electr. QE-12 (1976) pp. 532-539.

Lattice structures for DFB lasers are today predominantly made by holographic lithography, ie by exposure of a photoresist layer applied to the surface of the laser substrate made of semiconductor material in an optical interference field, developing the layer and etching of the surface covered with the developed photoresist layer, a relief-like lattice structure having a spatial frequency corresponding to the spatial frequency of the interference field being formed in the surface of the substrate. The remaining steps of laser production take place in a known manner.

The optical interference field is generated by optically superimposing two coherent light waves. However, this results in only simple lattice structures without a phase jump.

Electron beam recorders are currently used to produce lattice structures with phase jump (see, for example, K. Sekartedjo et al: 1.5 µm phase-shifted DFB lasers for single-mode operation, Electr. Lett. 20 (1984) pp. 80-81) or holographic Lithography method with combined use of positive and negative photoresist (see K. Utaka et al: λ / 4-shifted InGaAsP / InP DFB lasers by simultaneous holographic exposure of positive and negative photoresist, Electr. Lett. 20 (1984) p 1008-1010).

In the former case, a very expensive electron beam recorder is required; in the latter case, it is a technologically complex process that can lead to high failure rates in laser production.

In both cases, lattice structure fabrication is time consuming.

The object of the invention is to provide a particularly simple method for producing a grating structure with a phase shift, in particular grating structures for DFB lasers.

This object is achieved on the basis of a method of the type mentioned at the outset by the features specified in the characterizing part of patent claim 1.

The method according to the invention then differs from the currently usual holographic lithography for producing a lattice structure without a phase shift for DFB lasers essentially only by superimposed exposure of the photoresist layer in at least two optical interference fields of different spatial frequencies.

The method according to the invention can be used according to claim 2 in photoresist technology, but also according to claim 3 in a maskless laser-active etching technique (see, for example, RM Lum, FW Ostermayer Jr., PA Kohl, AM Glass, AA Ballman in Appl. Phys. Lett. 47 ( 3), Aug. 1985, pp. 269-271).

Three particularly expedient ways of carrying out the method according to the invention emerge from subclaims 4 to 6.

• Figur 1 eine bestimmte Belichtungsfunktion B(Z) zur Er­zeugung einer Gitterstruktur eines DFB-Lasers, wobei die auf diese Funktion bezogene minimale Länge L_min, optimale Länge L_opt und maximale Länge L_max des Lasers angegeben sind,
• Figur 2 die Belichtung einer Fotolackschicht auf der Ober­fläche des Substrats in dem optischen Interferenz­feld zweier optisch überlagerter ebener kohärenter Lichtwellen;
• Figur 3 die Belichtung der Fotolackschicht auf der Ober­fläche des Substrats in dem Interferenzfeld zweier optisch überlagerter divergenter kohärenter Lichtwellen,
• Figur 4 die Belichtung der Fotolackschicht auf der Ober­fläche des Substrats in dem Interferenzfeld einer ebenen Lichtwelle und einem mit dieser optisch überlagerten und von einem Spiegel umgelenkten Anteil dieser Welle, und
• Figur 5 die Belichtung einer mit einer durch Belichtung aktivierbaren Ätzflüssigkeit in Kontakt stehen­den Oberfläche des Substrats in dem Interferenzfeld einer ebenen Lichtwelle und einem mit dieser opti­sch überlagerten und von einem Spiegel umgelenkten Anteil dieser Welle.

The invention is explained by way of example in the following description with reference to the figures. From the figures show:

• 1 shows a specific exposure function B (Z) for generating a lattice structure of a DFB laser, the minimum length L _min , optimal length L _opt and maximum length L _{max of} the laser _relating to this function being indicated,
• FIG. 2 shows the exposure of a photoresist layer on the surface of the substrate in the optical interference field of two optically superimposed, plane, coherent light waves;
• FIG. 3 shows the exposure of the photoresist layer on the surface of the substrate in the interference field of two optically superimposed divergent coherent light waves,
• 4 shows the exposure of the photoresist layer on the surface of the substrate in the interference field of a plane light wave and a portion of this wave optically superimposed on it and deflected by a mirror, and
• 5 shows the exposure of a surface of the substrate which is in contact with an etching liquid which can be activated by exposure in the interference field of a plane light wave and a portion of this wave which is optically superimposed on it and deflected by a mirror.

In the following it is assumed, for example, that the photosensitive surface of the substrate, i.e. the photoresist layer or the surface of the substrate in contact with the etching liquid is double-exposed in an interference field, the spatial frequency K of the interference field being changed by a small amount 2ΔK between the individual exposures.

Dies ist eine Belichtungsfunktion mit der Ortsfrequenz K moduliert mit der Modulationsfunktion sin ΔK·Z. Der Vor­ zeichenwechsel der Modulationsfunktion an den Stellen ΔK·Z = N·π läßt sich auch als Phasensprung der modulier­ten Funktion an dieser Z-Werten interpretieren:

The exposure function B₁ (Z) of the first exposure, ie the location dependence of the light intensity in the Z direction on the photosensitive layer, for example, is approximately
B₁ (Z) = B₀ (1 + sin (K + ΔK) Z). (1)
The exposure function of the second exposure is
B₂ (Z) = B₀ (1 - sin (K - ΔK) Z). (2)
The exposure function of the total exposure is given by

This is an exposure function with the spatial frequency K modulated with the modulation function sin ΔK · Z. The before The change in the sign of the modulation function at the points ΔK · Z = N · π can also be interpreted as a phase jump of the modulated function at these Z values:

The exposure function B (Z) is therefore a grating function with phase jumps at a distance ΔZ = π / ΔK.

The exposed photoresist layer is developed and the lattice structure is transferred into the semiconductor material of the substrate by etching. The remaining steps of laser production are also carried out in a known manner.

gelten.The length L of the laser is chosen so that a phase jump falls in the middle of the laser and that L does not become larger than the period L _max = 2π / ΔK of the modulation function sinΔK · Z; on the other hand, the maximum modulation depth of the grating should still be achieved in the laser, that is, L should not be less than half the period L _{min of} the modulation function, that is

be valid.

The product L · K = L _opt · K is preferably chosen to be about 4.6.

For a given laser length L of, for example, approximately 500 μm, this results in a value of approximately three lines / mm for the spatial frequency change 2ΔK / 2π between the two exposures.

Three expedient and advantageous embodiments of the double exposure of the photosensitive surface of the substrate are described below with reference to FIGS. 2 to 5.

In these figures, the substrate is denoted by 1, its surface by 11, and a solder which has fallen onto the surface 11 by S. The plane of incidence in which the optically superimposed light waves are incident is the plane of the drawing in all FIGS. 2 to 5.

In FIGS. 2 to 4, the photosensitive surface of the substrate 1 consists of a photoresist layer 2 applied to the surface 11.

According to FIG. 2, two coherent plane waves 2, 3 incident from different directions R 1 and R 2 overlap in front of the photoresist layer 2 and form an interference field 34, which generates 2 parallel interference strips that run perpendicular to the plane of the drawing on this photoresist layer. The spatial frequency K of these interference fringes can be largely adjusted by the angle of incidence of the plane waves and their wavelength. The angle of incidence of the plane shaft 3 or 4 is known to be given by the angle α 1 or α 2 measured between the assigned direction R 1 or R 2 and the plumb line S.

The photoresist 2 is exposed to these interference fringes for a sufficient exposure time.

For the second exposure, the angle of incidence α 1 and / or α 2 of the plane waves 3 and 4 is changed. This is done most simply by rotating the substrate about an axis M perpendicular to the plane of incidence, for example in the direction of the arrow R₃, in which case the changed angles of incidence are given by α₁ + β and α₂ - β. This automatically changes the Spatial frequency K of the interference fringes on the photoresist layer 2. The required change in the spatial frequency ΔK can be set by the amount of the angle β by which the rotation takes place. The position of the axis of rotation M is relatively uncritical.

According to FIG. 3, two coherent divergent waves 5, 6 originating from source points or lines Q5, Q6 and coming in from different directions R₅ or R über overlap in front of the photoresist layer 2. They form an interference field 56, which likewise produces interference strips on the photoresist layer 2. At predetermined angles of incidence α₅ or α₆ of the divergent waves 5, 6, the spatial frequency K of these interference fringes can be largely adjusted by the distance d of the photoresist layer 2 from the source points Q5, Q6.

If the distance d is selected for the one exposure, this distance d is increased or also reduced for the second exposure by shifting in the direction R₇ parallel to the perpendicular S, for example. As a result, the spatial frequency of the interference fringes on the photoresist layer 2 changes automatically. The required spatial frequency change ΔK can be set by changing the distance Δd.

According to FIG. 4, the substrate 1 is firmly connected to a plane mirror Sp arranged at an angle δ of, for example, 90 °. From the direction R₈, a plane wave 8 falls on both the photoresist layer 2 and on the mirror Sp. The mirror Sp reflects the portion 18 of the plane wave 8 that hits it, for example half, in the direction of the photoresist layer 2. This creates in front of the photoresist layer 2 an interference field 80 which generates interference fringes on this layer, the spatial frequency K of which depends on the angle of incidence α₈ of the plane wave on the photoresist layer 2 and on the angle δ between the mirror Sp and the photoresist layer 2 or surface 11.

If the spatial frequency K is selected for the one exposure, the spatial frequency change ΔK for the second exposure can be set simply by rotating substrate 1 and mirror Sp together about an axis M ', for example in the direction of arrow R₉. As in the embodiment according to FIG. 2, the required change in spatial frequency .DELTA.K can be set by the amount of the angle .gamma. By which it is rotated. Here, too, the position of the axis of rotation M 'is relatively uncritical.

In FIG. 5, the photosensitive surface of the substrate 1 consists of its surface 11 itself, which is in contact with an etching liquid 20 that can be activated by exposure. Otherwise, the entire arrangement according to FIG. 5 corresponds to the arrangement according to FIG. 4, the corresponding elements having the same reference numerals . In contrast to FIG. 4, the arrangement according to FIG. 5 is enclosed in a transparent cuvette 10 containing the etching liquid 20.

The incident laser light 8 triggers a chemical reaction directly in the substrate material on the surface 11 of the substrate 1. The formation of the grating with a phase shift takes place in a very analogous manner as when using photoresist: in the first exposure step, for example, a grating with a first grating constant K = Λ + ΔΛ, the second exposure step a grating with the grating constants Λ - ΔΛ is generated. The lattice resulting from the superposition of these two lattices has an average lattice constant Λ which is modulated sinusoidally with the period ΔΛ.

All liquids and gases known for laser-active etching can be used as etchants. For example, a mixture of one part of H₂SO₄, one part of H₂O₂ and 10 parts of H₂O (see above-mentioned reference Appl. Phys. Lett.) Can be used, for example is suitable for a substrate material made of InP.

The same optical arrangement for exposure can be used for the production of the lattice structures by laser-active etching and the production by means of photoresist. Thus, in the method according to FIG. 5, instead of the arrangement according to FIG. 4, the arrangement according to FIG. 2 or FIG. 3 can also be used.

Reference symbol list

• B (Z) exposure function
• L _min minimum length of the laser
• L _max maximum length of the laser
• L _opt optimal length of the laser
• 1 Substat
• 11 surface of the substrate
• 2 photoresist layer
• S solder precipitated on the surface
• R₁, R₂ different directions
• 3, 4 coherent plane waves
• 34 interference field
• K spatial frequency of the interference fringes
• α₁, α₂ angle of incidence of the plane waves
• M axis perpendicular to the plane of incidence
• R₃ arrow
• β angle by which the substrate 1 is rotated about the axis M.
• Q₅, Q₆ source points or lines
• R₅, R₆ incident divergent waves
• 5, 6 coherent divergent waves
• α₅, α₆ angle of incidence of the divergent waves 5, 6
• d Distance of the photoresist layer from the source points Q₅, Q₆
• R₇ direction parallel to Lot S
• Δd distance change
• δ angle at which the mirror Sp is arranged relative to the substrate 1
• Sp mirror
• R₈ direction
• 8 flat shaft
• 18 part of the plane wave 8 reflected by the mirror Sp
• 80 interference field
• α₈ angle of incidence of the plane shaft 8
• M 'axis
• R₉ arrow
• γ angle by which to rotate
• 20 etching liquid
• 10 cuvette
• Λ lattice constant
• ΔΛ period with which the lattice constant is modulated

Claims (6)

1. A method for producing a lattice structure with phase jump on the surface of a substrate (1) by exposure of the photosensitive surface (2, 11) in an optical interference field (34, 56, 80) and developing the exposed surface, characterized by superimposed exposure of the photosensitive Surface (2, 11) in at least two interference fields (34, 56, 80) of different spatial frequencies. 2. The method according to claim 1, characterized in that the photosensitive surface of the substrate (1) consists of a on the substrate (1) applied photoresist layer (2), which is developed after the superimposed exposure, the surface covered with the developed photoresist layer (11) of the substrate is etched with an etchant which attacks the surface (11) of the substrate. 3. The method according to claim 1, characterized in that the photosensitive surface consists of the substrate surface (11) in contact with an etchant (20), on which the etching reaction can be activated by exposure. 4. The method according to any one of the preceding claims, characterized in that the photosensitive surface (2, 11) in the interference field (34) of two plane coherent waves (3, 4) is exposed at different angles of incidence. 5. The method according to any one of claims 1 to 3, characterized in that the photosensitive surface (2, 11) in the interference field (56) two divergent coherent light waves (5, 6) at different distances from the source points or lines (Q₅, Q₆) of the divergent waves (5, 6) is exposed. 6. The method according to any one of claims 1 to 3, characterized in that the photosensitive surface (2, 11) in the interference field (80) of a plane wave (8) and one of a relative to the substrate (1) fixed mirror (Sp) deflected portion (18) of this wave is exposed at different angles of incidence.

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